The invention relates to the alignment of the axis of multi-body systems, including but not limited to multi-body systems for the polishing of semiconductor wafer surfaces. More specifically, the present invention relates to a method and system for dynamic self-alignment of the axis of rotation of a semiconductor wafer surface and a polishing surface.
Semiconductor wafers are commonly constructed in layers, where a portion of a circuit is created on a first level and conductive vias are made to connect up to the next level of the circuit. After each layer of the circuit is etched on the wafer, an oxide layer is put down allowing the vias to pass through but covering the rest of the previous circuit level. Each layer of the circuit can create or add unevenness to the wafer that must be smoothed out before generating the next circuit layer.
Chemical mechanical polishing (CMP) techniques are used to polish and planarize the raw wafer and each layer of circuitry added. Available CMP systems, commonly called wafer polishers, often use a rotating wafer carrier head that brings the wafer into contact with a polishing pad rotating in the plane of the wafer surface to be planarized. A chemical polishing agent or slurry containing microabrasives is applied to the polishing pad to polish the wafer. The wafer carrier head then presses the wafer against the rotating polishing pad and is rotated to polish and planarize the wafer. The mechanical force for polishing is derived from the rotating table speed and the downward force on the wafer carrier head.
A conventional way to axially align the rotating polishing pad with the wafer carrier is to use a rigid surface on one body, such as a body that supports the polishing pad, and a gimbal on the other. A problem occurs when the polishing surface of the rotating polishing pad is smaller than the wafer surface, and especially as the polishing surface moves off an edge of the wafer during polishing. Since the gimbaled surface tends to tilt at the edge, alignment of the wafer to the polishing surface becomes difficult, and is preferably accomplished by using all rigid surfaces. Many polishing techniques, such as CMP, however, require at least one dynamic self-aligning surface to align the polished layer to a previously processed underlying layer of the wafer.
Thus, there is a need for a method for polishing wafers where the wafer is dynamically aligned to a polishing surface and then rigidly held in place when the alignment is accomplished.
A method and system are disclosed for automatically aligning and setting an axis of rotation of a semiconductor wafer to a polishing pad, for example, without using a gimbal mechanism usually incorporated into a wafer head, i.e., wafer carrier. After an angle of the axis of rotation is aligned to the pad, the angle is fixed in place. In this manner, the polishing pad can effectively polish a semiconductor wafer that is attached to the wafer head.
According to an aspect of the invention, a polishing pad includes a first surface and a semiconductor wafer includes a second surface. To polish the semiconductor wafer, axis of rotation of the first and second surfaces are aligned to each other. To allow axial alignment of the surfaces, at least one of the first and second surfaces includes an adjustable axis of rotation. After the axis of rotation of the first and second surfaces is aligned, the adjustable axis of rotation is set to maintain the adjusted position. Thereafter, the polishing pad polishes the semiconductor wafer, for example, in a radially symmetric fashion.